| Document and Entity Information | May 05, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 5, 2021 |
| Entity Registrant Name | CSG SYSTEMS INTERNATIONAL, INC. |
| Entity Central Index Key | 0001005757 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, Par Value $0.01 Per Share |
| Trading Symbol | CSGS |
| Security Exchange Name | NASDAQ |
| Entity File Number | 0-27512 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 47-0783182 |
| Entity Address, Address Line One | 6175 S. Willow Drive |
| Entity Address, Address Line Two | 10th Floor |
| Entity Address, City or Town | Greenwood Village |
| Entity Address, State or Province | CO |
| Entity Address, Postal Zip Code | 80111 |
| City Area Code | 303 |
| Local Phone Number | 200-2000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
